Electrostatic printing method and apparatus



J. MATKAN 3,355,288

ELECTROSTATIC PRINTING METHOD AND APPARATUS Nov. 28, 1967 2 SheetsSheet 1 Filed Nov. 1'7, 1964 Nov. 28, 1967 J. MATKAN 3,3555288 ELECTROSTATIC PRINTING METHOD AND APPARATUS Filed Nov. 17, 1964 2. Sheets-Sheet 2 United States Patent 3,355,288 ELECTROSTATIC PRINTING METHOD AND APPARATUS Josef Matkan, Malvern, South Australia, Australia, assignor to Research Laboratories of Australia Pty. Limited, North Adelaide, South Australia, Australia Filed Nov. 17, 1964, Ser. No. 411,795

Claims priority, application Australia, Nov. 19, 1963,

37,824/ 63 7 Claims. (Cl. 96-1) This invention relates to improvements in electrostatic printing, and in particular it relates to electrostatic printing of the type in which a master carries an insulating image thereon which image can accept an electrostatic charge and which insulating and charged image is then supplied with developer which is selectively attracted and deposited by reason of the existence of the charge and the deposit is then transferred to paper or the like.

With this type of printing certain advantages exist in that the master can for instance be placed on a conductive cylinder which can be revolve-d past a charging station and then through a developer station and the image deposit can then be transferred electrostatically to a web of paper or the like, it being customary with this type of operation to utilise electrical attraction of the developer to the master and to use a bias which will control the rate of deposition of this developer. The developers may comprise an insulating carrier liquid having a volume resistivity in excess of ohm centimetre and a dielectric constant of less than 3, having suspended therein particulate toner material capable of being attracted and deposited electrostatically.

It has been found however that certain difficulties exist in the correct development of the charge-bearing insulated sections of the master and the subsequent transfer of the developed image to the web of paper or the like and it is the object of this invention to overcome any such problems.

The principal problem encountered is the avoidance of distortion of the transferred image through the various stages. A secondary problem is avoidance of background deposition.

It has now been found that in addition to the electrical requirements the critical factors which grovern good transfer are the control of the volume of liquid present at both the developing stat-ion and the transfer station, and it is found that this can be controlled by four factors the first being a rigid control at the developer station of the volume of liquid which is applied to the master, the second being the location of the transfer station in relation to the applicator station to ensure that the required volume of developer liquid is carried to the transfer station, the third being to try and achieve as near as possible a single line contact at the transfer station to avoid image rupture, shear or distortion, and the fourth being a rigid control of the pressure at the transfer station between the paper or the like to which transfer is effected and the master.

The advantages of the invention are achieved firstly by applying the liquid developer to the master by means of rotating rollers so arranged that there is no contact between any moving parts and the master of this locality and that sufficient gap is left between the rollers and the master to give adequate development of the image and application of the liquid developer thereto without however causing undue shear which would limit the volume of liquid left on the surface of the master and which would distort or dislocate the image deposit thereon.

This developing station can therefore conveniently comprise a series of rollers which dip into the developer and which form a trough between them adjacent to the sur- 3,355,288 Patented Nov. 28, 1967 face of the master, which can conveniently be on a cylinder in this case, the rollers however being spaced sufficiently from the cylinder say .01 of an inch to leave a gap through which the developer can enter and leave the trough.

The transfer station is preferably located as close as possible to the applicator position and in a case of a cylinder should preferably be within an arc of 30 degrees to either side from the horizontal centre line of the cylinder after it leaves the developing station so that the developed image reaches the transfer station at a short but controlled time after development.

To achieve as near as possible a single line contact, the transfer roller should be substantially smaller than the master cylinder, and it is generally found that a transfer roller which is generally /3 to /3 the diameter of the cylinder is satisfactory, the minimum diameter of the roller however preferably being not less than a quarter of an inch for the reason that diificulty can otherwise exist in passing the paper or the like around it and also it is obviously necessary to ensure that the paper or the like has its surface moving at the exact linear speed of the master so as to avoid distortion of the image such as that caused by slipping occurring if the speeds of the paper and of the master cylinder are not the same.

The pressure of the roller is so selected that a film of liquid developer will be maintained between the paper or the like passing round the roller and the master, and if desired the paper or the like can actually be lifted from the surface of the master by mechanical adjustment means or by suitably shaping the roller or the master such as by including ridges at the edges, to give the required spacing, it being essential however that any gap which is left at this locality be such that during the transfer this gap is maintained full of liquid so that the electrical transfer can take place in an efficient manner.

In connection with this transfer it should be pointed out that when applying liquid developer to an insulating image on a master carrying an electrostatic charge which attracts the developer particles, the field which causes the deposition of the toner particles should act through the carrier liquid in which the particles are suspended and the same applies at the transfer station and it is therefore essential to ensure that both the field which applies the toner particles to the master and the field which transfers the toner particles from the master to the paper or the like act through a body of the carrier liquid for the particles so that correct surface charge of the particles is maintained in relation to the field.

To enable the invention to be fully appreciated an embodiment of same will now be described with reference to the accompanying drawings in which:

FIG. 1 is a somewhat schematic view showing the relationship of the various integers of the invention,

FIG. 2 is a fragmentary enlarged section showing the operating principle of the invention, and

FIG. 3 shows how the rollers may be driven.

In both FIGS. 1 and 2 the gaps between rollers and the master have been magnified to show the action which takes place.

The cylinder 1 has on it a master 2 which can conveniently comprise a sheet of paper on which is coated a photocon-ductive film preferably comprising particulate zinc oxide embedded in an insulating resin matrix the master having an insulating image thereon which can be formed by depositing an insulating material such as a resin on the photoconductive surface and permanently fixing it thereon by solvent evaporation or heat-fusing or the like.

The master thus comprises insulating image sections 3 between which the photoconductor surface is exposed so that when the master is charged a charge will be held on the insulating image areas but the non-image areas can be readily discharged where the photoconductor is exposed to a light source 19.

The charging of the insulating image areas is effected by means of a high voltage discharge between a transfer roller 4 and the cylinder 1, the voltage being generated by any of the well known forms of high voltage generators 5, this voltage being used also to transfer the image from the cylinder to a paper web 6 or other transfer material.

The developer is applied to the master 2 by rollers 7, S and 9 at a developer station, at least the lower roller 9 being disposed clear of the master 2 with a gap of perhaps 0.01 of an inch so that while the rollers ensure adequate supply of developer to the master, there is no actual contact between the rollers and the master. The gaps are designated 13 and 14.

To provide a bias at the developer station which will ensure that background contamination by adsorption does not take place on the relatively conductive non-image areas formed on the photoconductor surface, a bias voltage is applied between the cylinder 1 and the roller 9, which bias has the same polarity as the charge on the particles so that the particles are urged away from the biased sections of the master and therefore do not adhere thereto.

The bias can conveniently be applied through a potentiometer 15, the slider 16 of which is adjustable near one end of the potentiometer so that the actual voltage applied between the cylinder and the roller 9 can be selected, this voltage being only of a low order, such as for instance 5030O volts.

The gap 17 forms the charging and transfer area and must only be sufliciently wide to ensure that there is a thin film of liquid in same.

Separate generators could be used for the high voltage charging and for the biasing instead of the arrangement indicated.

In FIG. 2 is shown how the invention operates. The insulating image areas 3 of the master carry the charge which is impressed by the roller 4, acros the gap 17, so that as the master passes through the developer station the liquid developer in the channel 12 is applied to the face of the master 2 and the toner particles are then attracted to the insulating image areas because of their inherent charge in relation to the impressed charge on the insulating image areas 3, which of course is opposite to the charge on the developer particles, and this liquid developer is carried around on the master to the transfer roller 4 where transfer of the image takes place, not by contact between the master and the web 6 but by the field which is applied across the gap 17 between the master 2 and the outer face of the web of paper 6.

Thus the developed image is lifted electrically on to the web 6 at the transfer locality, this being possible because sufficient of the developer is retained on the face of the master to bridge the gap 17 between the surface of the paper web 6 and the surface of the master 2.

It will therefore be seen that according to this invention there is at no time contact between the master and either the developer applicator rollers 7 and 8 or the web 6 there being as said a gap 13 between the roller 7 and the surface of the master 2 through which liquid developer is carried into the channel 12 and similarly there is a gap 14 between the roller 8 and the surface of the master 2 so that while liquid developer is applied to the surface of the master, there is no wiping and consequently no smudging on the master because of the absence of physical contact.

By adjusting the distance of the rollers 7 and 8 it is obvious that a close control of the application of the developer can be achieved and a film of developer can be left adhering to the surface of the master 2 which will give adequate carrying over of the developer to the transfer station at the roller 4.

To further control the thickness of the liquid developer film adhering to the surface of the master, the transfer roller 4 should be so positioned that such liquid film is not diminished in thickness by drainage to such extent that the gap 17 at the transfer station is not filled with such liquid. The preferred position of the transfer roller 4 has been found to be within 30 degrees to either side of the horizontal centre line of the master cylinder 1 after it leaves the developing station as indicated in FIGS. 1 and 3.

At the transfer station there is the gap 17 between the web 2 on which the transfer is taking place and the surface of the master 2 so that again there is no smuudging of the image or spreading as would be the case if there was actual physical contact, and the fact that the developer liquid is carried to this locality ensures that the toner particles, which are carried on the insulating image areas 3 to which they are attracted during development, retain their correct polarity which is governed by their existence in the carrier liquid of the developer, the par ticles therefore transferring effectively at the transfer locality because they are up to this stage retained in the environment of the carrier liquid of the developer.

The attracted developer is designated 18.

To give as close a line contact as possible the roller 4 should not be greater than one third the diameter of the drum but preferably would be only about one eighth of this diameter provided that for reasons of mechanical rigidity its minimum diameter should not be lower than approximately one quarter of an inch.

The light 19 serves to discharge the electrostatic charge which may otherwise remain on the non-image areas on the photoconductor of the master 2.

While development of the image is normally completed in the area between the rollers 7 and 8, toner particles existing in the liquid developer film in the area 20 between the rollers 3 and 4 move to the charge-carrying insulating image areas 3 of the master so that an extremely clean image results from the use of the invention.

Referring to FIG. 3 the cylinder 1 is driven by a belt 22 from a pulley 23 on a motor or the like, the belt being of a thickness such that the roller 4 when in contact with it under influence of the spring 24 will have the correct gap between the surface of the master 2 and the surface of the web 6 passing over the roller 4.

In this way the gap is automatically provided by a selection of the thickness of the belt, there preferably being a belt on each end of the cylinder 1, and the cylinder 1 and roller 4 are driven at their correct peripheral speeds.

A pulley 25 on the shaft 26 of the pulley 23 has a belt 27 driving a pulley 23 on a shaft 29 which carries the roller 9 of FIGS. 1 and 2 within the tank 11.

In this way a simple drive is provided for the various mechanisms, the rollers 7 and 8 of course being driven by contact with the roller 9. The shaft 29 of the roller 9 is connected to earth as shown in FIG. 1 to provide a return for the potentiometer 15 through the slider 16 which is also connected to earth, the position of the slider 16 fixing the voltage between the conductive cylinder 1 and the shaft 9 and thus the voltage acting to repel toner particles from the conductive areas of the master 2.

At least roller 9 is preferably made of metal to provide the electrode within the tank 11.

It will be realised that as the developer is placed into contact with the master 2 the repulsion voltage on nonimage areas of the master 2 can be selected by the position of the slider 16 to closely control the extent of propulsion of the pigment particles away from the master in the non-image areas and it will be realised also that by appropriately spacing the rollers 7 and 8 the volume of developer fed into the channel 12 and the volume of developer liquid allowed to remain on the master 2 can be closely regulated so that the exact volume of carry-over of developer liquid to the gap 17 can be achieved.

The gap 17 can of course be formed by lightly loading the roller 4 towards the cylinder 1 so that the developer liquid itself keeps the web 6 the required distance from the surface of the master 2 to allow transfer through the thin liquid film.

It will be realised that when toner particles are suspended in an insulating carrier liquid, the charge on the particles themselves is governed by the characteristics of the particles and of the liquid in which they are suspended and therefore for the purpose of this invention it is desirable to ensure that at any transfer locality this body of liquid is present to hold the particles at their correct charge value and to thus allow effective transfer of the particles either to the insulating image areas of the master or from these areas to the pap-er or other transfer material.

1 From the foregoing it will be realised that the present invention consists firstly in the use of applicator means so arranged that a required body of liquid is left on the master, and secondly that a body of liquid is maintained at the transfer locality through which electrical transfer takes place, the transfer mechanism being so arranged that as near as possible to line contact or proximity results for the purpose as earlier stated of preventing the rupture, shear or distortion of the image at the transfer locality.

It will be realised of course that the charging of the insulated surface can be carried out simultaneously with the transferring of previously applied developer to the sheet of paper or the like and that a discharge light can be used for those areas where the charge is not to be held in the case of the master being formed on a photoconductive sheet.

With regard to the formation of the master it can be stated that this can conveniently be produced by charging a photoconductor surface, then light-modifying the surface to produce the desired electrostatic pattern, and to then develop this with a resin or other insulating medium which will leave the image on the master in the form of an insulating pattern which can accept an electrostatic charge and hold it in the presence of light whilst the nonimage areas of such master cannot retain such charge in the presence of radiation capable of increasing the conductivity of the photoconductor.

What I claim is:

1. An improved method of effecting electrostatic printing in which a master having insulating and relatively non-insulating sections thereon is used which method consists of first subjecting the master to a corona discharge to charge the insulating sections, then passing the charged master through a developer applicator station where liquid developer comprising an insulating carrier liquid having suspended therein toner particles capable of being electrostatically attracted and deposited is projected against the surface of the master without contact between the applicator mechanism and the master, then moving the master to a transfer station while maintaining a film of liquid developer on the master, and then transferring the developed image from the master to a receiving member spaced from the face of the master to leave a gap while such receiving member is carried around a roller so positioned that the required gap is maintained and the receiving member is kept in line proximity with the image bearing master and at such position in relation to the developing station that a liquid film remains on the surface of the master at the transfer station, maintaining the gap full of developer liquid carried by the master, and applying an electrical field through the developer liquid in the gap to transfer the developed image from the master to the receiving member.

2. An improved method of effecting electrostatic printing in which a master having insulating and relatively non-insulating sections is carried on a cylinder, and while the cylinder is rotating subjecting the master to a corona discharge to charge the insulating sections, then passing the charged areas through a developer station and applying liquid developer against the face of the master without mechanical contact between the applicator mechanism and the master, then transporting the developed master to a transfer station while maintaining a film of liquid developer on the master, and then transferring the developed image from the master to a receiving member carried around a roller spaced from the face of the master to leave a gap between the face of the master and the receiving station which gap is filled by liquid developer carried on the master and app-lying an electrical field through the developer in the gap to transfer the developed image from the master to the receiving member.

3. An improved method of effecting electrostatic printing in which a master having a photoconductor surface thereon carrying an insulating section is carried on a cylinder, and while the cylinder is rotating subjecting the master to a corona discharge to charge the insulating image bearing sections, then discharging the non-insulating sections of the photoconductor surface by light, then passing the charged areas through a developer station and applying liquid developer against the surface of the master without mechanical contact between the applicator mechanism and the master, then moving the developed master to a transfer station while maintaining a film of liquid developer on the master, and then passing the developed image from the master to a receiving member carried around a roller spaced from the face of the master to leave a gap between the surface face of the master and the receiving member wherein such gap is filled by developer liquid, and applying an electrical field through the developer in the gap to transfer the developed image from the master to the receiving member.

4. A machine for effecting electrostatic printing comprising a cylinder adapted to contain a master having insulating and relatively conducting sections forming an image, a combined charging and transfer station adjacent the cylinder including a transfer roller over which a web of material which is to receive the final print is passed, said spacing of the roller in relation to the master on the cylinder being such as to leave a gap between the web and the surface of the cylinder to be filled by developer carried by the said master, high voltage generating means connected to supply a charging voltage between the roller and the cylinder, a developer station adjacent the cylinder comprising rollers adjacent the cylinder to lift developer to the cylinder but not in contact with the master thereon, means to supply developer to the said rollers, means to drive the cylinder and the developer rollers, and means to drive the transfer roller to hold the web at the same peripheral speed and direction as the master on the said cylinder.

5. A machine for effecting electrostatic printing comprising a cylinder adapted to contain a master having insulating and relatively conducting sections forming an image, a combined charging and transfer station adjacent the cylinder including a transfer roller over which a Web of material which is to receive the final print is passed, said spacing of the roller in relation to the master on the cylinder being such as to leave a gap between the web and the surface of the cylinder to hold a film of developer liquid carried by the master and said roller being positioned in relation to the cylinder so that its horizontal axis is Within thirty degrees of the horizontal centerline of the master cylinder, high voltage generating means to apply a charging current between the roller and the cylinder, and a developer station adjacent the cylinder comprising rollers adjacent the cylinder but not in contact with the master thereon, means to supply developer to the said rollers, means to drive the cylinder, means to drive the transfer roller to cause the web to move in the same direction at the same peripheral speed as the master on the cylinder, and means to drive the developer rollers to lift developer against the master on the cylinder, the gap between the developer rollers and the cylinder being such that developer is carried from the developer station to the transfer station by the master on the cylinder whereby printing of the image on the master is effected by liquid developer applied to the surface of the master and transfer is similarly effected by electrical transfer through a film of liquid between the surface of the master and the web.

6. A machine for effecting electrostatic printing comprising a cylinder adapted to contain a master having insulating and relatively conducting sections forming an image, charging means adjacent the cylinder, high voltage generating means connected to apply a charging current between the charging means and the cylinder, a developer station adjacent the cylinder comprising rollers adjacent the cylinder to lift developer to the cylinder but not in contact with the master thereon, means to supply developer to the said rollers, means to drive the cylinder and the developer rollers, a transfer station including a transfer roller over which a web of material which is to receive the final print is passed, said transfer roller being so positioned that its horizontal axis is within 30 degrees of the horizontal centerline of the master cylinder, the spacing of the roller in relation to the master on the cylinder being such as to leave a gap between the web and the surface of the Cylinder to be filled by developer carried by the said master, means to apply a transfer voltage through the gap between the Web and the surface of the cylinder, and means to drive the transfer roller to hold the web at the same peripheral speed and direction as the master on the said cylinder.

7. A machine for effecting electrostatic printing comprising a cylinder adapted to contain a master having insulating and relatively conducting sections forming an image, a combined charging and transfer station adjacent the cylinder including a transfer roller over which a web of material which is to receive the final print is passed, said spacing of the roller in relation to the master on the cylinder being such as to leave a gap between the web and the face of the cylinder to hold a film of developer liquid carried by the master, high voltage generating means to apply a charging current between the roller and the cylinder, and a developer station adjacent the cylinder comprising rollers adjacent the cylinder but not in contact with the master thereon, means to supply developer to the said rollers, means to drive the cylinder, means to drive the transfer roller to cause the Web to move in the same direction at the same peripheral speed as the master on the cylinder, and means to drive the developer rollers to lift developer against the master on the cylinder, the gap between the developer rollers and the cylinder being such that developer is carried from the developer station to the transfer station by the master on the cylinder whereby printing of the image on the master is effected by liquid developer applied to the surface of the master and transfer is similarly effected by electrical transfer through a film of liquid between the surface of the master and the web.

References Cited UNITED STATES PATENTS 2,637,651 5/1953 Copley 961.4 3,203,395 8/1965 Liller 1l8637 3,232,190 1/1966 Willmott 96l 3,271,146 9/1966 Robinson 961.4 3,284,224 11/1966 Lehmann l17--l7.5

NORMAN G, TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner; 

3. AN IMPROVED METHOD OF EFFECTING ELECTROSTATIC PRINTING IN WHICH A MASTER HAVING A PHOTOCONDUCTOR SURFACE THEREON CARRYING AN INSULATING SECTION IS CARRIED ON A CYLINDER, AND WHILE THECYLINDER IS ROTATING SUBJECTING THE MASTER TO A CORONA DISCHARGE TO CHARGE THE INSULATING IMAGE BEARING SECTIONS, THEN DISCHARGING THE NON-INSULATING SECTIONS OF THE PHOTOCONDUCTOR SURFACE BY LIGHT, THEN PASSING THECHARGED AREAS THROUGH A DEVELOPER STATION AND APPLYING LIQUID DEVELOPER AGAINST THE SURFACE OF THE MASTER WITHOUT MECHANICAL CONTACT BETWEEN THE APPLICATOR MECHANISM AND THE MASTER, THEN MOVING THE DEVELOPED MASTER TO A STRANSFER STATION WHILE MAINTAINING A FILM OF LIQUID DEVELOPER ON THE MASTER, AND THEN PASSING THE DEVELOPED IMAGE FROM THE MASTER TO A RECEIVING MEMBER CARRIED AROUND A ROLLER SPACED FROM THE FACE OF THE MASTER TO LEAVE A GAP BETWEEN THE SURFACE FACE OF THE MASTER AND THE RECEIVING MEMMBER WHEREIN SUCH GAP IS FILLED BY DEVELOPER LIQUID, AND APPLYIN AN ELECTRICAL FIELD THROUGH THE DEVELOPER IN THE GAP TO RANSFER THE DEVELOPED IMAGE FROM THE MASTER TO THE RECEIVING MEMBER.
 5. A MACHINE FOR EFFECTING ELECTROSTATIC PRINTING COMPRISING A CYLINDER ADAPTED TO CONTAIN A MASTER HAVING INSULATING AND RELATIVELY CONDUCTING SECTIONS FORMING AN IMAGE, A COMBINED CHARGING AND TRANSFER STATION ADJACENT THE CYLINDER INCLUDING A TRANSFER ROLLER OVER WHICH A WEB OF MATERIAL WHICH IS TO RECEIVE THE FINAL PRINT IS PASSED, SAID SPACING OF THE ROLLER IN RELATION TO THE MASTER ON THE CYLINDER BEING SUCH AS TO LEAVE A GAP BETWEEN THE WEB AND THE SURFACE OF THE CYLINDER TO HOLD A FILM OF DEVELOPER LIQUID CARRIED BY THE MASTER AND SAID ROLLER BEING POSITIONED IN RELATION TO THE CYLINDER SO THAT ITS HORIZONTAL AXIS IS WITHIN THIRTY DEGREES OF THE HORIZONTAL CENTERLINE OF THE MASTER CYLINDER, HIGH VOLTAGE GENERATING MEANS TO APPLY A CHARGING CURRENT BETWEEN THE ROLLER AND THE CYLINDER, AND ADEVELOPER STATION ADJACENT THE CYLINDER COMPRISING ROLLERS ADJACENT THE CYLINDER BUT NOT IN CONTACT WITH THE MASTER THEREON MEANS TO SUPPLY DEVELOPER TO THE SAID ROLLERS, MEANS TO DRIVE THE CYLINDER, MEANS TO DRIVE THE TRANSFER ROLLER TO CAUSE THE WEB TO MOVE IN THE SAME DIRECTION AT THE SAME PERIFPHERAL SPEED AS THE MASTER ON THE CYLINDER, AND MEANS TO DRIVE THE DEVELOPER ROLLERS TO LIFT DEVELOPER AGAINST THE MASTER ON THE CYLINDER, THE GAP BETWEEN THE DEVELOPER ROLLERS AND THE CYLINDER BEING SUCH THAT DEVELOPER IS CARRIED FROM THE DEVELOPER STATION TO THE TRANSFER STATION BY THE MASTER ON THE CYLINDER WHEREBY PRINTING OF THE IMAGE ON THE MASTER IS EFFECTED BY LIQUID DEVELOPER APPLIED TO THE SURFACE OF THE MASTER AND TRANSFER IS SIMILARLY EFFECTED BY ELECTRICAL TRANSFER THROUGH A FILM OF LIQUID BETWEEN THE SURFACE OF THE MASTER AND THE WEB. 